How to test if preprocessor symbol is #define'd but has no value?

Question

Using C++ preprocessor directives, is it possible to test if a preprocessor symbol has been defined but has no value? Something like that:

#define MYVARIABLE #if !defined(MYVARIABLE) || #MYVARIABLE == "" ... blablabla ... #endif 

EDIT: The reason why I am doing it is because the project I'm working on is supposed to take a string from the environment through /DMYSTR=$(MYENVSTR), and this string might be empty. I want to make sure that the project fails to compile if user forgot to define this string.

Answer 1

Soma macro magic:

#define DO_EXPAND(VAL)  VAL ## 1 #define EXPAND(VAL)     DO_EXPAND(VAL)  #if !defined(MYVARIABLE) || (EXPAND(MYVARIABLE) == 1)  Only here if MYVARIABLE is not defined OR MYVARIABLE is the empty string  #endif 

Note if you define MYVARIABLE on the command line the default value is 1:

g++ -DMYVARIABLE <file> 

Here the value of MYVARIABLE is the empty string:

g++ -DMYVARIABLE= <file> 

The quoting problem solved:

#define DO_QUOTE(X)        #X #define QUOTE(X)           DO_QUOTE(X)  #define MY_QUOTED_VAR      QUOTE(MYVARIABLE)  std::string x = MY_QUOTED_VAR; std::string p = QUOTE(MYVARIABLE); 

Answer 2

I haven't seen this solution to the problem but am surprised it is not in common use . It seems to work in Xcode Objc. Distinguish between "defined with no value" and "defined set 0"

#define TRACE #if defined(TRACE) && (7-TRACE-7 == 14) #error TRACE is defined with no value #endif